Silicon carbide (SiC) has the following characteristics. An insulation break-down electric field of SiC is larger than that of silicon (Si) by one digit, a band gap of SiC is as large as three times a band gap of Si, and thermal conductivity of SiC is as high as three times the thermal conductivity of Si. Accordingly, application to a power device, a high frequency device, a high-temperature operation device, and the like has been expected. According to this, recently, a SiC substrate has been used as a substrate of a semiconductor device.
For example, the above-described SiC substrate is manufactured from a SiC bulk single crystal ingot that is prepared by a sublimation method and the like. Typically, the SiC substrate is obtained by the following procedure. That is, the outer periphery of the ingot is processed to have a cylindrical shape, the ingot is sliced into a disk shape by using a wire saw and the like, and an outer peripheral portion of the sliced article is chamfered to be finished with a predetermined diameter. In addition, a grinding process is performed with respect to a surface of the disk-shaped SiC substrate in accordance with a mechanical grinding method so as to adjust unevenness and a degree of parallelization. Then, mechanochemical polishing such as a chemical mechanical polishing (CMP) method is performed with respect to the surface, and thus one surface or both surfaces are mirror-finished. The grinding and polishing with respect to the SiC substrate are performed for planarization of the surface of the SiC substrate, and the like in addition to removal of waviness or work strain which occurs due to the slicing.
The above-described CMP method is a polishing method including both a chemical operation and a mechanical operation, and thus it is possible to stably obtain a flat surface without damaging the SiC substrate. Accordingly, in a process of manufacturing a SiC semiconductor device and the like, the CMP method is widely employed as a method of planarizing waviness on a surface of the SiC substrate or unevenness on a wafer, in which an epitaxial layer is laminated on the surface of the SiC substrate, due to interconnection and the like.
In addition, a wafer in which the SiC substrate is used is manufactured by growing a SiC epitaxial film which becomes an active region of a SiC semiconductor device on the SiC substrate obtained in the above-described procedure by chemical vapor deposition (CVD). On the other hand, in a case of using the SiC substrate that is sliced from the SiC single crystal ingot in a state in which unevenness or waviness occurs on the surface, unevenness and the like also occur on a surface of an epitaxial layer that is formed on the surface of the SiC substrate. Accordingly, when manufacturing a wafer in which a SiC epitaxial film is grown on the SiC substrate, the surface of the SiC substrate is polished in advance by using the CMP method, and even after the growth of the SiC epitaxial film, a grinding process according to the mechanical grinding method, and finish polishing according to the CMP method are performed so as to planarize the substrate surface, that is, the wafer surface.
Here, in a case where the epitaxial layer is grown on the surface of the SiC substrate in a state in which waviness or work strain remains thereon, and a semiconductor element such as a transistor and a diode is formed on the epitaxial layer so as to manufacture a semiconductor device, it is difficult to obtain electrical characteristics which are expected from original excellent physical properties of SiC. Accordingly, the process of planarizing the surface of the SiC substrate as described above becomes a very important process.
Generally, as a process of removing the waviness or work strain on the surface of the SiC substrate, for example, it is effective to use a mechanical polishing method such as lap polishing. In addition, with regard to planarization of the surface, for example, polishing using diamond having a particle size of 1 μm or less, or grinding using a grinding stone with a high count of #10000 or greater is effective. In addition, as finish processing on the surface of the SiC substrate before growth of the SiC epitaxial film, or as finish processing of the wafer after the SiC epitaxial film is formed, polishing in accordance with the CMP method is generally performed so as to obtain surface roughness Ra of less than 0.1 μm.
A method of grinding the surface of the SiC substrate with the CMP method in the related art will be described with reference to FIGS. 6 and 7.
As illustrated in FIG. 6, a SiC substrate 100, whose surface is ground with the mechanical grinding method after being sliced, is mounted to a rotatable SiC substrate-supporting portion 201 that is provided to a CMP polisher 200. Then, the SiC substrate 100 is pressed to a polishing pad 202a that is attached to a surface of a rotary platen 202, and the SiC substrate-supporting portion 201 is rotated while supplying slurry 204 to an interface between the polishing pad 202a and the SiC substrate 100 from a slurry nozzle 203, thereby polishing a polishing surface (surface) 100a of the SiC substrate 100.
However, even when planarizing the SiC substrate 100 in accordance with the method of the related art, at an initial stage of a process of polishing the SiC substrate 100 with the polishing in accordance with the CMP method, as illustrated in FIG. 6, scratches 300 occur on the polishing surface 100a. This is because at the initial stage of the CMP polishing, the scratches 300 tend to occur on the substrate surface due to an operation of pressing the polishing surface 100a of the SiC substrate to the polishing pad 202a, and a rotating operation of the SiC substrate 100 that is mounted to the SiC substrate-supporting portion 201. Here, in a case of CMP polishing the SiC substrate 100, generally, an average particle size of colloidal silica that is used as slurry is approximately 0.2 μm to 0.5 μm, and from a relationship between the amount removed by the CMP polishing and remaining scratches, it is assumed that the depth of the scratches 300 is approximately 0.5 μm or less. In a case where the scratches 300 occur due to the CMP polishing, as illustrated in FIG. 7, even after the SiC substrate 100 is detached from the SiC substrate-supporting portion 201, the scratches 300 remain on the polishing surface 100a, and thus there is a problem in that a yield ratio decreases.
Here, with regard to an improvement in surface roughness that is an original object during polishing of the SiC substrate 100 with the CMP method, it is possible to accomplish the improvement in a polishing time of approximately 10 minutes. On the other hand, when the scratches 300 occur at an initial stage of the polishing in accordance with the CMP method, even though the improvement in the surface roughness is accomplished, a new additional process is necessary to remove the scratches which occur at an initial processing stage. Typically, during the polishing in accordance with the CMP method, a processing rate is slow in comparison to other methods, and thus in a case of performing the additional process as described above, a processing time that is taken to remove the scratches 300 is added in a unit of several hours, and thus there is a problem that the process time is lengthened.
As a method of polishing the surface of the SiC substrate in accordance with the CMP method, for example, the following technology is suggested. Specifically, a plurality of SiC substrates are mounted on a rotary table, and when the SiC substrates are subjected to the CMP polishing with a batch process, the thickness of the substrate before the polishing is adjusted after applying a liquid material to a surface opposite to the polishing surface of each of the SiC substrates, and the thickness of the respective SiC substrates is adjusted without occurrence of mechanical damage such as the scratches on the substrate surface (refer to PTL 1). According to the technology that is disclosed in PTL 1, the above-described method is employed, and thus the thickness of the plurality of substrates can be adjusted without the mechanical damage on the substrate, and thus it is possible to obtain an effect capable of suppressing a polishing amount from being different on each surface of the plurality of SiC substrates.
In addition, the following technology is also suggested. Specifically, the SiC substrate is suctioned and fixed onto a rotary table, and when the SiC substrate is subjected to grinding, a curable material is laminated in advance on both surfaces of the SiC substrate, and the above-described grinding is performed after curing the curable material (for example, refer to PTL 2). According to the technology that is disclosed in PTL 2, a curable layer composed of a curable material is provided on both surfaces of the SiC substrate, and thus substrate rigidity is increased. Accordingly, when the SiC substrate is suctioned and fixed to a rotary table during the subsequent grinding process, it is possible to suppress the occurrence of waviness on the SiC substrate, and thus it is possible to prevent the waviness from remaining on the SiC substrate after the grinding.